Local Flashcards
When are local anaesthetics used
- When loss of consciousness is neither necessary or desirable
- As an adjunct to surgery to avoid high-dose general anaesthetics
- Post operative analgesia
- For major surgery with sedation
What type of local anaesthetics are most commonly used
Amide containing with ester containing rarely used
What are the amide containing locals
- Lidocaine
- Prilocaine
- Bupivacaine
- Articaine
What is lidocaine
Medium acting, rapid onset
What is prilocaine
Medium acting, no vasodilation
What is bupivacaine
Long acting, slow onset
What is articaine
Short acting, rapid onset
What are the ester containing locals
- Tetracaine
- Chloroprocaine
- Benzocaine
What is tetracaine
Long acting, very slow onset
What is chloroprocaine
Medium acting
What is benzocaine
Atypical mechanism of action
How do locals work
By reversibly blocking voltage-gated Na+ channels. This stabilises excitable membranes and prevents membranes from being depolarised
What are the factors affecting the probability that a LA will block an impulse
- Diameter of the fiber - smaller blocked more easily
- Myelination status - myelinated fibers blocked more easily
- Length of nerve exposed to drug
- Length of time exposed to drug
- Conc of drug
Describe the blocking of voltage-gated Na channels by LAs
- LAs are weak bases
- They can exist as neutral or protonated
- pH dependant
- Block when ionised, via an intracellular binding site
- But can’t pass through membrane when ionised
What is pKa
The pH at which 50% of drug is ionised. Most LAs have pKa values between 8 and 9
What is the Henderson-Hasselbalch equation
For a weak base:
pKa - pH = log10 [BH+]/[B]
What do most LAs cause
Vasodilation except cocaine (vasoconstriction) and prilocaine
What is duration of action dictated by
Rate of removal:
- Decreased blood flow to area therefore decreased rate of removal
- Vasoconstriction can increase duration 2x, decrease bleeding during surgery
What is eutectic mixture of LAs (EMLA) used for
Dermal anaesthesia
Describe EMLA
- Mixture of lignocaine and prilocaine
- Melting point of mixture is lower than that of the individual chemicals
- Therefore higher conc of both can be used
- More chance more will get across the skin
What are the side effects of EMLA entering systemic circulation
- Dysrhythmias
- Sudden fall in BP
- Restlessness
- Tremors
